From the first day in the police academy, every cop on the force has the mantra “Police! Don’t move,” and the protocol for facing down the barrel of a gun drilled into their heads on a near-daily basis.

In a situation where a plainclothes officer – whether off duty or undercover – hears that clarion call directed at him or her from another officer’s mouth, the patrol guidebook says they are supposed to do exactly what is ordered and explain later.

Officers encountering a suspect or a possible crime in progress are supposed to yell “Police! Don’t move,” unless they feel their life is in imminent danger. Then they are allowed to fire without warning.

The three-word mantra is so ubiquitous within the department that stickers bearing the phrase are placed on locker doors in precinct houses around the city.

Police brass drill the protocol home to avoid such tragic cop-on-cop shootings like the one that felled off-duty Officer Eric Hernandez in The Bronx yesterday morning.

How Hernandez and Officer Freddie Toro interacted in the shooting is not entirely clear, but police say at least one witness told them Toro clearly yelled at Hernandez to “drop your weapon.”

What Hernandez did next is not known yet, but Toro followed by shooting Hernandez three times, once in the abdomen and in each leg.

Hernandez was clinging to life in “grave condition” at St. Barnabas Hospital, Police Commissioner Ray Kelly said.